1. Field of the Invention
This invention relates to apparatus for conveying sheet materials, and particularly for conveying substantially electrically-non-conductive sheet materials such as plastics films or textile sheets.
2. Description of the Related Art
One particular application is in the field of folding machines for laundries, wherein hot sheets from a drying/ironing machine pass to a three-conveyor folding assembly. Each of the conveyors consists of a plurality of parallel, spaced apart belts entrained about rollers. The belts are usually of textile material and, the arriving sheets being hot and dry operate under hot and dry conditions. This, plus the high speeds, such as 40 m per minute, of the belts, create ideal conditions for static electricity to build up on the surfaces of the belts. In addition, the sheets from the dryer/ironer also travel quickly and are hot and dry. Thus, both the belts and the material being conveyed, ie the sheets, can and usually do, become highly charged with static electricity.
As is well known, the charge which develops on the surface of a particular material is characteristic of the material, and that material with which it is in frictional contact, in accordance with the Triboelectric series which is reproduced below.
______________________________________ TRIBOELECTRIC SERIES ______________________________________ Positive Wool Nylon 5. Viscose Cotton Silk Acetate Perspex 10. Polyvinyl Alcohol Polyester Acrylic Polypropylene P V C 15. Acrylic co-Polymer Polyvinylidene Chloride Leather Polythene Teflon 20. Negative ______________________________________
For example, it will be apparent that cotton fabric (cotton being higher than polyester in the series) can develop a positive charge under certain circumstances whereas polyester fabric could develop a negative charge under the same circumstances. It might be assumed from this that a polyester/cotton blend could be found which developed no charge. Unfortunately, this is not true, any blend always has a slight bias towards negative or positive, and once the charge appears it increases quickly.
In the aforesaid folding machine, it has been common practice to use belts made of cotton. Sheets, over the years, have usually also been made of cotton and there has been no problem with static electricity. This is a result of the nature of the materials and the nature of the machine. At a fold nip of the machine, it is desirable that a sheet passes off an end of a top conveyor and hangs free alongside a lower adjacent conveyor until its mid-point is level with the fold nip between the two belts. When this occurs, a jet of air is activated to blow the centre of the sheet into the fold nip. If the belts and the sheets are of the same polarity of static, ie both negative in this case, there is a repulsive force at the nip which aids in keeping the sheet out of the nip. If, however, the sheet and the belts are of opposite polarities there is an attraction which can cause a hanging sheet end to enter the fold nip prematurely. It will be appreciated that with belts and sheets both of cotton, no problems arose in the past. However, with the advent of polyester fibre sheets and blends thereof, belts and sheets have been of opposite static polarities and have caused attraction at the fold nip with consequent premature folding. It will be appreciated that if sheets could be sorted into cotton and polyester types, and each folded on a separate machine, no problem would arise. However, such sorting is time consuming and expensive, and is often impractical, for example in a laundry having only one folding machine. Additionally, the aforesaid problem can cause delay and difficulty if the problem is not diagnosed rapidly, because a machine operating perfectly on one sort of sheet may suddenly begin to malfunction when a different sort of sheet passes through it, which sheet might be visually indistinguishable from the rest. This can cause stoppages and delays whilst some imagined fault is investigated.
It has been proposed to use belt fabrics which are highly conductive, in that they contain metallic or carbon filaments intended to dissipate electrostatic charges to earth in the machine. This does not work, especially with very fast machines, as the remainder of the fabric in the belt becomes charged and cannot dissipate the charge quickly enough.
An object of the present invention therefore, is to provide an improved belt for conveying substantially electrically-non-conductive material and an improved conveyor for this purpose.